Ink jet printing mechanisms use pens that shoot droplets of colorant onto a printable surface to generate an image. Such mechanisms may be used in a wide variety of applications, including computer printers, plotters, copiers, and facsimile machines. For convenience, the concepts of the invention are discussed in the context of a printer. An ink jet printer typically includes a print head having a multitude of independently addressable firing units. Each firing unit includes an ink chamber connected to a common ink source, and to an ink outlet nozzle. A transducer within the chamber provides the impetus for expelling ink droplets through the nozzles. In thermal ink jet printers, the transducers are thin film firing resistors that generate sufficient heat during application of a brief voltage pulse to vaporize a quantity of ink sufficient to expel a liquid droplet.
The energy applied to a firing resistor affects performance, durability, and efficiency. It is well known that the firing energy must be above a certain threshold to cause a vapor bubble to nucleate. Above this threshold in a transitional range, increasing the energy increases the drop volume expelled. Above a higher threshold at the upper limit of the transitional range, drop volumes do not increase with increasing energy. It is in this range in which drop volumes are stable even with moderate energy variations that printing ideally takes place, as variations in drop volume cause disuniformities in printed output. As energy levels increase above this optimal zone, uniformity is not compromised, but energy is wasted, and the printer components are prematurely aged due to excessive heating and ink residue build up.
In existing systems having a dedicated connection for each firing resistor, a one time calibration of each connection by printer or production circuitry external to the pen also compensates for any parasitic resistance or impedance in the unique path leading to each resistor. Print heads may be characterized at production to set these operating parameters.
However, in highly multiplexed print heads having different sets of nozzles, each set addressed by a common voltage line, there may be variations due to other factors. Each set of nozzles is powered by a single voltage line that receives power via an electrical contact pad between the printer electronics and the removable print cartridge. This line continues on a flex circuit to a tab bonding connection to the print head die having other electronics, including the firing resistors. The impedance of the print cartridge contacts, tab bonding connections, and connections in between can vary from cartridge to cartridge, from nozzle to nozzle, and over time, even when the voltage provided by the printer to each of the cartridge contacts is well controlled. Consequently, as printed data changes, the current draw through the line and the voltage as measured at the pen terminals may be undesirably varied. For instance, when many or all nozzles are fired simultaneously, the pen voltage may be depressed by parasitic effects, giving a lower firing energy than when only one or a few nozzles are fired.
The present invention overcomes the limitations of the prior art by providing a method of operating a thermal ink jet printer with a removable print head having a plurality of ink firing resistors, the operation includes calibrating the printer by determining a nominal input voltage above a threshold necessary for simultaneous operation of a plurality of the resistors. Then, during printing, detecting the input voltage on the print head at an input node connected to the resistors and generating a firing pulse having a duration based on the detected input voltage at the node. Thus, a detected input voltage higher than the nominal voltage is compensated for by a shortened firing pulse. The method may be achieved in a removable ink jet print head having a connector with many electrical inputs connectable to a printer, and a voltage input node connected to the connector. The print head has numerous firing resistors connected to the input node. An energy control circuit is connected to the input node and is connected to the resistors to fire the resistors with a firing pulse having a duration inversely related to the voltage at the input node, such that the firing energy is maintained in control by varying the firing pulse duration to compensate for voltage variations.